How reproducibly can human ear ossicles be measured? A study of inter-observer error.

Ear ossicles have thus far received little attention in biological anthropology. For the use of these bones as a source of biological information, it is important to know how reproducibly they can be measured. We determined inter-observer errors for measurements recorded by two observers on mallei (N = 119) and incudes (N = 124) obtained from human skeletons recovered from an early medieval cemetery in southern Germany. Measurements were taken on-screen on images of the bones obtained with a digital microscope. In the case of separately acquired images, mean inter-observer error ranged between 0.50 and 9.59% (average: 2.63%) for malleus measurements and between 0.67 and 7.11% (average: 2.01%) for incus measurements. Coefficients of reliability ranged between 0.72 and 0.99 for the malleus measurements and between 0.61 and 0.98 for those of the incus. Except for one incus measurement, readings performed by the two observers on the same set of photographs produced lower inter-observer errors and higher coefficients of reliability than the method involving separate acquisition of images by the observers. Across all linear measurements, absolute inter-observer error was independent of the mean size of the measured variable for both bones. So far, studies on human ear ossicles have largely neglected the issue of measurement error and its potential implication for the interpretation of the data. Knowledge of measurement error is of special importance if results obtained by different researchers are combined into a single database. It is, therefore, suggested that the reproducibility of measurements should be addressed in all future studies of ear ossicles.